President Yoon Suk Yeol delivers a public address from his official residence in Seoul, Saturday, following the National Assembly's passage of an impeachment motion against him. Courtesy of presidential office

Investigators are expected to expedite a probe into President Yoon Suk Yeol on charges of treason and abuse of power, after his impeachment by the National Assembly on Saturday.

As the anti-corruption investigation body has signaled the possibility of detaining or arresting Yoon, attention is focused on whether he could become the first sitting president in Korea to face arrest.

Prosecutors made the first move by summoning the president to appear for questioning in the martial law probe. But Yoon defied the request, they said.

"We notified President Yoon Suk Yeol on Wednesday to appear at 10 a.m. today (Sunday) for questioning. But he did not show up. We will make a second request," the prosecutors' special investigation headquarters said.

The impeachment motion against Yoon was centered on allegations that his abrupt declaration of martial law earlier this month constitutes treason. Lawmakers highlighted that the declaration lacked both substantial and procedural constitutional and legal grounds.

According to the legal community, treason charges are categorized into three levels of culpability — the leader who orchestrates and directs the treasonous activity, key operatives involved in planning and commanding and general participants who carry out or support the treasonous activity without a clear role.

Ongoing investigations have so far targeted primarily high-ranking military and police officials suspected of playing key roles in the president's botched martial law declaration.

With Yoon now suspended from duty, investigators will likely focus on the president, who could face charges as the mastermind who directed the insurrection.

Officials from the National Office of Investigation (NOI) carry boxes of confiscated evidence and forensic equipment as they exit the civil service office at the presidential office in Seoul, Wednesday. The NOI conducted a search of the presidential office as part of its investigation into the martial law incident. However, it received only limited documents through voluntary submission after being denied entry by the Presidential Security Service. Yonhap

How will the investigation proceed?

Uncertainties remain over how the investigation into the president will proceed and which agency will take charge.

Discussions are ongoing about the possibility of arresting him or obtaining a warrant to trace his whereabouts if he refuses to comply.

On Wednesday, Oh Dong-woon, chief of the Corruption Investigation Office for High-ranking Officials (CIO), said the agency could pursue an emergency arrest or a court-approved warrant if legal conditions are met.

Despite the impeachment motion being passed, potential clashes with the presidential security service remain a challenge, as Yoon retains full security and protocol privileges until a formal dismissal.

There is also the increasing possibility of a search and seizure targeting Yoon's communication records and his Seoul residence.

The National Office of Investigation (NOI) attempted to enter the presidential office last Wednesday to secure documents and materials related to Yoon's martial law declaration. However, the search was cut short as the Presidential Security Service did not comply with the notice, citing the need for prior consultation. Instead, the police were provided with limited documents by the security service.

Therefore, authorities may also attempt to conduct another search and seizure at the presidential office.

Currently, the probe into the president's martial law declaration is being conducted by two main bodies — the prosecution's special investigation headquarters and the cooperation investigation headquarters comprised of the police, the CIO and the Ministry of National Defense's investigation headquarters.

Tensions have escalated as investigative agencies compete to secure witness statements first, leading to overlapping schedules.

Earlier, Lt. Gen. Kwak Jong-keun, head of the Army Special Warfare Command, and former National Intelligence Service Deputy Director Hong Jang-won were summoned by both the CIO and prosecutors on the same day and underwent separate questioning at each location.

Amid unclear boundaries over investigative authority, concerns have been raised that suspects, including Yoon, could exploit the situation by choosing the investigative agency they perceive as more favorable.

Lt. Gen. Yeo In-hyung, former chief of the Defense Counterintelligence Command, attends a meeting of the Intelligence Committee at the National Assembly in Yeouido, Seoul, Dec. 7. Yonhap

More arrests

Meanwhile, investigations into key figures involved in the martial law incident have gained momentum.

On Sunday, Kwak was accused of deploying troops to the National Assembly during the imposition of martial law on Dec. 3. He is also alleged to have colluded with the president and former Defense Minister Kim Yong-hyun to incite a riot aimed at subverting the Constitution.

Lt. Gen. Yeo In-hyung, former chief of the Defense Counterintelligence Command, was arrested on Saturday on charges of orchestrating operations to mobilize military and police forces to the National Assembly and the National Election Commission.

On Friday, the Seoul Central District Court issued warrants for Cho Ji-ho, commissioner general of the National Police Agency, and Kim Bong-sik, head of the Seoul Metropolitan Police Agency, citing concerns over the potential destruction of evidence.

Both are accused of ordering police officers to enter the National Assembly and prevent lawmakers from voting to repeal Yoon's martial law decree.

Former Defense Minister Kim, believed to be the mastermind behind the entire martial law fiasco, was formally arrested last Tuesday. Kim's legal counsel said Saturday that he would exercise his right to remain silent during questioning ahead of his scheduled appearance before prosecutors.